Millions of genetically modified mosquitoes could be released in the Florida Keys if British researchers win approval to use the bugs against two extremely painful viral diseases.

Never before have insects with modified DNA come so close to being set loose in a residential U.S. neighborhood.

"This is essentially using a mosquito as a drug to cure disease," said Michael Doyle, executive director of the Florida Keys Mosquito Control District, which is waiting to hear if the Food and Drug Administration will allow the experiment.

Dengue and chikungunya are growing threats in the U.S., but some people are more frightened at the thought of being bitten by a genetically modified organism. More than 130,000 people signed a Change.org petition against the experiment.

Even potential boosters say those responsible must do more to show that benefits outweigh the risks of breeding modified insects that could bite people.

"I think the science is fine, they definitely can kill mosquitoes, but the GMO issue still sticks as something of a thorny issue for the general public," said Phil Lounibos, who studies mosquito control at the Florida Medical Entomology Laboratory.

Derric Nimmo / AP

In September, a horrific T-bone crash in Tallahassee took the lives of three young South Floridians. Morgan Carr, 21, Vincenzo Libio, 22, and Kevin Boos, 21, were all from Broward County. Read the full story here:

In September, a horrific T-bone crash in Tallahassee took the lives of three young South Floridians. Morgan Carr, 21, Vincenzo Libio, 22, and Kevin Boos, 21, were all from Broward County. Read the full story here:

(Derric Nimmo / AP)

Mosquito controllers say they're running out of options. With climate change and globalization spreading tropical diseases farther from the equator, storm winds, cargo ships and humans carry these viruses to places like Key West, the southernmost U.S. city.

There are no vaccines or cures for dengue, known as "break-bone fever," or chikungunya, so painful it causes contortions. U.S. cases remain rare.

Insecticides are sprayed year-round in the Keys' charming and crowded neighborhoods. But Aedes aegypti, whose biting females spread these diseases, have evolved to resist four of the six insecticides used to kill them.

Enter Oxitec, a British biotech firm that patented a method of breeding Aedes aegypti with fragments of genes from the herpes simplex virus and E. coli bacteria as well as coral and cabbage. This synthetic DNA is commonly used in laboratory science and is thought to pose no significant risks to other animals, but it kills mosquito larvae.

Oxitec has built a breeding lab in Marathon and hopes to release its mosquitoes in a Key West neighborhood this spring.

FDA spokeswoman Theresa Eisenman said no field tests will be allowed until the agency has "thoroughly reviewed all the necessary information."

Company spokeswoman Chris Creese said the test will be similar in size to Oxitec's 2012 experiment in the Cayman Islands, where 3.3 million modified mosquitoes were released over six months, suppressing 96 percent of the targeted bugs. Oxitec says a later test in Brazil also was successful, and both countries now want larger-scale projects.

But critics accused Oxitec of failing to obtain informed consent in the Caymans, saying residents weren't told they could be bitten by a few stray females overlooked in the lab.

Instead, Oxitec said only non-biting males would be released, and that even if humans were somehow bitten, no genetically modified DNA would enter their bloodstream.

Neither claim is entirely true, outside observers say.

"I'm on their side, in that consequences are highly unlikely. But to say that there's no genetically modified DNA that might get into a human, that's kind of a gray matter," said Lounibos.

Creese says Oxitec has now released 70 million of its mosquitoes in several countries and received no reports of human impacts caused by bites or from the synthetic DNA, despite regulatory oversight that encourages people to report any problems. "We are confident of the safety of our mosquito, as there's no mechanism for any adverse effect on human health. The proteins are non-toxic and non-allergenic," she said.

Oxitec should still do more to show that the synthetic DNA causes no harm when transferred into humans by its mosquitoes, said Guy Reeves, a molecular geneticist at Germany's Max Planck Institute.

Key West resident Marilyn Smith wasn't persuaded after Oxitec's presentation at a public meeting. She says neither disease has had a major outbreak yet in Florida, so "why are we being used as the experiment, the guinea pigs, just to see what happens?"